Developmental cheating in the social bacterium Myxococcus xanthus

Cheating is a potential problem in any social system that depends on cooperation and in which actions that benefit a group are costly to individuals that perform them. Genetic mutants that fail to perform a group-beneficial function but that reap the benefits of belonging to the group should have a within-group selective advantage, provided that the mutants are not too common. Here we show that social cheating exists even among prokaryotes. The bacterium Myxococcus xanthus exhibits several social behaviours, including aggregation of cells into spore-producing fruiting bodies during starvation. We examined a number of M. xanthus genotypes that were defective for fruiting-body development, including several lines that evolved for 1,000 generations under asocial conditions and others carrying defined mutations in developmental pathways, to determine whether they behaved as cheaters when mixed with their developmentally proficient progenitor. Clones from several evolved lines and two defined mutants exhibited cheating during development, being overrepresented among resulting spores relative to their initial frequency in the mixture. The ease of finding anti-social behaviours suggests that cheaters may be common in natural populations of M. xanthus.     

Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress

The extension of growing season at high northern latitudes seems increasingly clear from satellite observations of vegetation extent and duration. This extension is also thought to explain the observed increase in amplitude of seasonal variations in atmospheric CO2 concentration. Increased plant respiration and photosynthesis both correlate well with increases in temperature this century and are therefore the most probable link between the vegetation and CO2 observations. From these observations, it has been suggested that increases in temperature have stimulated carbon uptake in high latitudes and for the boreal forest system as a whole. Here we present multi-proxy tree-ring data (ring width, maximum late-wood density and carbon-isotope composition) from 20 productive stands of white spruce in the interior of Alaska. The tree-ring records show a strong and consistent relationship over the past 90 years and indicate that, in contrast with earlier predictions, radial growth has decreased with increasing temperature. Our data show that temperature-induced drought stress has disproportionately affected the most rapidly growing white spruce, suggesting that, under recent climate warming, drought may have been an important factor limiting carbon uptake in a large portion of the North American boreal forest. If this limitation in growth due to drought stress is sustained, the future capacity of northern latitudes to sequester carbon may be less than currently expected.     

Lateral gene transfer and the nature of bacterial innovation

Unlike eukaryotes, which evolve principally through the modification of existing genetic information, bacteria have obtained a significant proportion of their genetic diversity through the acquisition of sequences from distantly related organisms. Horizontal gene transfer produces extremely dynamic genomes in which substantial amounts of DNA are introduced into and deleted from the chromosome. These lateral transfers have effectively changed the ecological and pathogenic character of bacterial species.     

CpG methylation is maintained in human cancer cells lacking DNMT1

Hypermethylation is associated with the silencing of tumour susceptibility genes in several forms of cancer; however, the mechanisms responsible for this aberrant methylation are poorly understood. The prototypic DNA methyltransferase, DNMT1, has been widely assumed to be responsible for most of the methylation of the human genome, including the abnormal methylation found in cancers. To test this hypothesis, we disrupted the DNMT1 gene through homologous recombination in human colorectal carcinoma cells. Here we show that cells lacking DNMT1 exhibited markedly decreased cellular DNA methyltransferase activity, but there was only a 20% decrease in overall genomic methylation. Although juxtacentromeric satellites became significantly demethylated, most of the loci that we analysed, including the tumour suppressor gene p16INK4a, remained fully methylated and silenced. These results indicate that DNMT1 has an unsuspected degree of regional specificity in human cells and that methylating activities other than DNMT1 can maintain the methylation of most of the genome.     

Bacterial photosynthesis in surface waters of the open ocean

The oxidation of the global ocean by cyanobacterial oxygenic photosynthesis, about 2,100 Myr ago, is presumed to have limited anoxygenic bacterial photosynthesis to oceanic regions that are both anoxic and illuminated. The discovery of oxygen-requiring photosynthetic bacteria about 20 years ago changed this notion, indicating that anoxygenic bacterial photosynthesis could persist under oxidizing conditions. However, the distribution of aerobic photosynthetic bacteria in the world oceans, their photosynthetic competence and their relationship to oxygenic photoautotrophs on global scales are unknown. Here we report the first biophysical evidence demonstrating that aerobic bacterial photosynthesis is widespread in tropical surface waters of the eastern Pacific Ocean and in temperate coastal waters of the northwestern Atlantic. Our results indicate that these organisms account for 2-5% of the photosynthetic electron transport in the upper ocean.     

Vascular-specific growth factors and blood vessel formation

A recent explosion in newly discovered vascular growth factors has coincided with exploitation of powerful new genetic approaches for studying vascular development. An emerging rule is that all of these factors must be used in perfect harmony to form functional vessels. These new findings also demand re-evaluation of therapeutic efforts aimed at regulating blood vessel growth in ischaemia, cancer and other pathological settings.     

Nicastrin modulates presenilin-mediated notch/glp-1 signal transduction and betaAPP processing

Nicastrin, a transmembrane glycoprotein, forms high molecular weight complexes with presenilin 1 and presenilin 2. Suppression of nicastrin expression in Caenorhabditis elegans embryos induces a subset of notch/glp-1 phenotypes similar to those induced by simultaneous null mutations in both presenilin homologues of C. elegans (sel-12 and hop-1). Nicastrin also binds carboxy-terminal derivatives of beta-amyloid precursor protein (betaAPP), and modulates the production of the amyloid beta-peptide (A beta) from these derivatives. Missense mutations in a conserved hydrophilic domain of nicastrin increase A beta42 and A beta40 peptide secretion. Deletions in this domain inhibit A beta production. Nicastrin and presenilins are therefore likely to be functional components of a multimeric complex necessary for the intramembranous proteolysis of proteins such as Notch/GLP-1 and betaAPP.     

Origins of sulphate in Antarctic dry-valley soils as deduced from anomalous 17O compositions

The dry valleys of Antarctica are some of the oldest terrestrial surfaces on the Earth. Despite much study of soil weathering and development, ecosystem dynamics and the occurrence of life in these extreme environments, the reasons behind the exceptionally high salt content of the dry-valley soils have remained uncertain. In particular, the origins of sulphate are still controversial; proposed sources include wind-blown sea salt, chemical weatherings, marine incursion, hydrothermal processes and oxidation of biogenic sulphur in the atmosphere. Here we report measurements of delta18O and delta17O values of sulphates from a range of dry-valley soils. These sulphates all have a large positive anomaly of 17O, of up to 3.4/1000. This suggests that Antarctic sulphate comes not just from sea salt (which has no anomaly of 17O) but also from the atmospheric oxidation of reduced gaseous sulphur compounds, the only known process that can generate the observed 17O anomaly. This source is more prominent in high inland soils, suggesting that the distributions of sulphate are largely explained by differences in particle size and transport mode which exist between sea-salt aerosols and aerosols formed from biogenic sulphur emission.